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The world’s 7.6 billion people only make up 0.01% of the Earth’s biomass. Yet since the rise of civilisation, humanity has caused the loss of more than 80% of all wild mammals and half of plants, while livestock has increased dramatically, new research shows. The study, published on May 21 in the journal “Proceedings of the National Academy of Sciences of the United States of America” (PNAS), provides a comprehensive assessment of the world’s living organisms and reveals our disproportionate impact on the Earth’s biosphere. To compare the biomass of bacteria to plankton to that of termites, trees, animals and humans, scientists at the Weizmann Institute of Science in Israel and the California Institute of Technology evaluated carbon units, for each group – measured in gigatonnes. They found that the sum of the biomass across all taxa on Earth is 550 Gt C, of which 450 Gt C or roughly 82% are plants, dominated by land plants. The second largest group is bacteria with 70 Gt C or 13%, followed by fungi (12 Gt C), archaea (7 Gt C), and protists (4 Gt C). Animals just make up 2 Gt C, of which humans represent only 0.06 Gt C, corresponding to 0.01% of the total biomass in the atmosphere. An animal with biomass that comes close to that of humanity – 0.05 gigatons of carbon – is the termite

The study also shows the enormous impact of humanity on all this biomass. The researchers compared their new estimates with those for the time before humans became farmers. Their results show that since the dawn of civilisation, humanity has destroyed 83% of wild mammals, 80% of marine animals, 50% of plants, and 15% of fish. “We know that humans affect the biosphere,” said Prof Ron Milo from the Weizmann Institute, “but now we are able to start showing the real numbers – to quantify our impact.” The authors write that “over the relatively short span of human history, major innovations, such as the domestication of livestock, adoption of an agricultural lifestyle, and the Industrial Revolution, have increased the human population dramatically and have had radical ecological effects.”

One effect is the rapid increase in the domesticated livestock biomass. According to the study, the biomass of humans and that of livestock (0.1 Gt C), dominated by cattle and pigs, far surpasses that of wild mammals (0.007 Gt C). 60% of all mammals on Earth are livestock, mostly cattle and pigs, 36% are human and just 4% are wild animals today. When it comes to birds, domesticated poultry (mostly chickens) constitute 70% of all birds on the planet, with just 30% being wild. “It is pretty staggering,” said Milo. “In wildlife films, we see flocks of birds, of every kind, in vast amounts, and then when we did the analysis we found there are [far] more domesticated birds.” He added that our dietary choices have a vast effect on the habitats of animals, plants and other organisms. “It is definitely striking, our disproportionate place on Earth,” Milo told the Guardian. “When I do a puzzle with my daughters, there is usually an elephant next to a giraffe next to a rhino. But if I was trying to give them a more realistic sense of the world, it would be a cow next to a cow next to a cow and then a chicken.” He hopes that the results of the study will affect the way people consume. “I have not become vegetarian, but I do take the environmental impact into my decision making, so it helps me think, do I want to choose beef or poultry or use tofu instead?” (ab)